Long deep vertical bioreactor systems suitable for the treatment of waste water by activated sludge processes are known and disclosed as for example, in U.S. Pat. No. 4,279,754 to Pollock.
A deep vertical bioreactor reactor system for the treatment of waste water, typically, comprises a bioreactor, a solid/liquid separator and intervening apparatus in communication with the bioreactor and separator. As fully described in aforesaid U.S. Pat. No. 4,279,754 such bioreactors essentially comprise a circulatory system which includes at least two substantially vertical side-by-side chambers in communication with each other at their upper and lower ends, with their upper ends being connected through a basin. The waste water for treatment is caused to circulate repeatedly through and between the downflow chamber (the downcomer) and the upflow chamber (the riser). Normally, the waste-containing liquor, referred to as mixed liquor, is driven through the circulating system injection of an oxyen-containing gas, usually air, into one or both of the chambers. Typically, in a 500 feet deep reactor air injection is at a depth of about 200 feet with the air at a pressure of 100 pounds per square inch. At start-up of the bioreactor a mixture of air and influent waste water is injected into the riser in the nature of an air lift pump. However, once circulation of the liquor begins, air injection can be also into the downcomer. The fluid in the downcomer having a higher density than the liquid-bubble mixture of the riser, thereby provides a sufficient lifting force to maintain circulation. Usually the basin is fitted with a baffle to force mixed liquor at the maintain circulation. Usually the basin is fitted with a baffle to force mixed liquor at the top of the riser to traverse a major part of the basin releasing spent gas before again descending the downcomer for further treatment.
Influent waste water is introduced at depth into the riser chamber through an upwardly directed outlet arm of an influent conduit. An oxygen-containing gas, usually air, is injected into the influent liquor in the outlet arm of the influent liquor conduit. In addition to oxygenating the waste liquor, the injected gas acts to create an air lift pump which draws the influent waste into the bioreactor riser. Effluent liquor is withdrawn from the riser through an effluent liquor conduit having its inlet located in the riser at a point below the outlet of the influent liquor conduit. During operation of the bioreactor the flow of influent liquor to and effluent liquor from the bioreactor are controlled in response to changes in level of liquid in the connecting upper basin.
The injected oxygen-containing gas dissolves in the mixed liquor as the liquor descends in the downcomer to regions of greater hydrostatic pressure. This dissolved oxygen constitutes the principal reactant in the biochemical degradation of the waste. As the circulating mixed liquor ascends in the riser to regions of lower hydrostatic pressure the dissolved gas separates and forms bubbles. When the liquid/bubble mixture from the riser enters the basin, gas disengagement occurs.
Reaction between waste, dissolved oxygen, nutrients and biomass substantially takes place during circulation through the downcomer, riser and basin bioreactor system. The products of the reaction are carbon dioxide, and additional biomass which in combination with unreacted solid material present in the influent waste water forms a sludge.
The term "Waste water" as used herein is understood to include water carrying any type of biodegradable domestic and industrial waste materials, for example, normal domestic waste and the effluents produced by farms, food factories, refineries, pulp mills, breweries and other industries. By "mixed liquor" is meant the mixture of liquids and solids present in the bioreactor system.
The term "dispersed gas bubbles" or "dispersed gas" in this application is quantitatively defined as the volume of gas in mL per litre of liquor that will spontaneously evolve from a sample of mixed liquor taken from a long shaft bioreactor when the sample is allowed to stand undisturbed for one minute. The term "dissolved gas" is quantitatively defined in this application as the volume of gas in mL per litre of liquor that will evolve from a sample of mixed liquor taken from a long shaft bioreactor after the sample has stood undisturbed for one minute and is then stirred under specific conditions for up to ten minutes minus the volume of dispersed gas that evolved after the first minute.
The measurement of the two types of gas may be done on a test rig provided for the purpose, as hereinafter described.
Effluent liquor withdrawn from the bioreactor comprises a mixture of liquid and solids commonly called sludge. Before the treated liquid component can be discharged into a natural water course, the solids component, i.e. sludge, must be separated. Separation is commonly carried out in a separation vessel by a combination of flotation and sedimentation. The gas bubbles which cause the solid particles in the mixed liquor to float to the liquid surface in the flotation vessel originate from the oxygen-containing gas which is dissolved in the mixed liquor as it circulates through the bioreactor. This dissolved gas comes out of solution in the form of bubbles as the mixed liquor rises to levels of lower hydrostatic pressure. Thus, when the effluent liquor stream reaches the surface it will contain dispersed gas bubbles which have already come out of solution, as well as dissolved gas remaining in the liquor, which is thus in a supersaturated state. Effective flotation of the solids in the sludge required that a brief period of about 1-4 minutes of gentle mixing of the sludge be provided to allow the particles to flocculate and form larger but more fragile agglomerates. The flocculated agglomerated particles are mixed with a stream of liquid containing supersaturated dissolved gas, which gas at the reduced pressure of the separation vessel spontaneously nucleates and generates micro-bubbles of gas to enhance flotation of the sludge flocculated particles. Balancing the conditions to obtain good flocculation against the requirements to provide microbubbles from the dissolved gas as quite difficult and at best a compromise in operating conditions. In practice, the addition of expensive flocculating agents such as cationic polymer is used to strengthen and maintain the floc integrity under the more turbulent conditions required for dissolved gas micro-bubble generation.
Energy levels are, generally, quantitatively defined by the concept of velocity gradients "G" expressed in sec.sup.-1 and is a measure of mixing intensity. Formulas are given in standard text books on waste water treatment e.g. . . . "Flocculation" is defined as the natural tendency for colliding particles to stick together to form agglomerates. Healthy biological solid particles have adhesive-like surface coating which increases the mechanical strength of the formed flocculant agglomerate.
During flocculation the destabilized particles grow and agglomerate to form large, settleable flocs. Through gentle prolonged mixing, chemical bridging, physical enmeshment of particles, or both occur. Flocculation is slower and more dependent on time and agitation than in a rapid mix step, such as seen in the bioreactor.
Energy levels for optimum flocculation of biosolid is generally around 100 sec.sup.-1. The addition of synthetic polymers to the mixed liquor may allow the floc to survive energy levels of 100-200 sec.sup.-1.
There is need for an improved bioreactor-separation apparatus and process which reduces the amount of flocculating agents necessary to provide good flocculation while providing improved dissolved gas micro-bubble generation.